Radar level gauges are in wide use for measuring the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby transmitted electromagnetic signals are radiated towards the product contained in a tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby transmitted electromagnetic signals are guided towards and into the product by a probe acting as a waveguide.
Radar level gauges are often classified as either pulsed system or FMCW-systems. In FMCW-systems, a signal with varying frequency is transmitted towards the surface and the distance is determined based on the frequency (and/or phase) difference between a transmitted signal and a simultaneously received signal. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. For a linear sweep, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal. The IF-signal will also comprise frequency components from reflections in the antenna and similar near-zone echoes. These near-zone echoes are very strong due to them occurring close to the transceiver, and thus making it hard to detect the surface echo. Hence, radar level gauges have filters which filter the IF-signal prior to any sampling, thereby suppressing frequency components which are not related to the surface in order to get good measurement data.
Radar level gauges are in many cases used for applications where malfunction of the radar level gauge could result in dangerous situations, and therefore radar level gauges must be extremely reliable. Various measures are taken to ensure the reliability of radar level gauges, and to thereby reduce the risk of dangerous situations. For instance, by performing a self-diagnostic function the radar level gauge may be proof tested to ensure that it is working properly. One way to perform a self-diagnostic function is to measure a reference echo, this will proof test the components of a radar level gauge which are related to generating, guiding, filtering, amplifying, transmitting and/or receiving electromagnetic signals, also known as the microwave chain.
U.S. Pat. No. 5,614,911 describes a FMCW radar level gauge where problems such as the formation of a deposit on the antenna, or other trouble such as damage or loss of the antenna may be detected. The problems are detected by first storing an undisturbed reference function, i.e. a tank spectrum, before the radar level gauge is put into actual operation. During actual operation, the measured echo function is compared to the stored undisturbed echo function and any differences are analyzed and evaluated to recognize the formation of deposits or other trouble. Similarly, US patent application 2006/0015292 is a time-domain-reflectometry (TDR) radar level gauge which may detect accretion of material on the antenna or malfunction of the electronics of the TDR radar level gauge. A constant measurement of the distance to a surface over a predetermined time interval is used to recognize accretion of material on the antenna or malfunction of the electronics of the TDR radar level gauge such as a short-circuit in the coupling between the antenna and the electronics. However, as antenna echoes are different for each antenna and as they degrade and change over time they are not suitable to use for a diagnostic functionality.